The present invention generally relates to the field of standby alternating current uninterruptible power supplies. More particularly, the invention is directed to an uninterruptible power supply (UPS) in which AC line power is used to operate a load and to charge a stand-by battery during a stand-by mode of operation of the UPS. When AC line power is not available and the UPS is operating in a back-up mode, the battery is used to supply power to the load.
The present invention differs from prior art UPSs in a number of ways, including the use of a single light weight, low cost, high frequency fly-back transformer in the power converter rather than the approach taken by prior art UPSs which rely on low (power line) frequency transformers. Low frequency transformers are heavy and more expensive than high frequency transformers.
There are numerous examples in the prior art of the use of low frequency transformers in stand-by uninterruptible power supplies. Such power supplies are described in U.S. Pat. Nos. 5,010,469; 4,916,329; 4,827,150 and 4,728,808. The disadvantages of the UPSs described in the foregoing patents include the use of a low frequency transformer, which as pointed out above, is large and expensive, in comparison with the remainder of the power supply circuitry.
U.S. Pat. No. 4,634,888, issued in the name of Deavenport, describes a UPS in which a pair of high frequency transformers are used. One transformer, together with its power converter, is used to charge the battery during stand-by operation. The second transformer, together with its power converter, is used to discharge the same battery for operating a load during back-up operation. The need for two power transformers and two corresponding power converters make the UPS described by the Deavenport patent expensive and overly complex to manufacture in comparison with the UPS of the present invention.
U.S. Pat. No. 4,277,692, issued in the name of Small, describes a UPS which uses a bi-directional power converter operating along with a low frequency push-pull transformer and common mode choke. While such a power supply is lower in cost than the Deavenport UPS, the use of a choke and low frequency transformer still adds to the cost and wight of the UPS.